All practical backscatter X-ray imaging systems are raster scanners, which acquire an image pixel by pixel while moving a well-collimated X-ray beam (also referred to as pencil beam) across the object under inspection. Typically, the sweeping X-ray beam is formed by mechanically moving an aperture in a line in front of a stationary X-ray source. The line is typically a straight line, or nearly so, such that an emergent beam sweeps within a plane, over the course of time. That plane is referred to as a “beam plane.” As the aperture moves along its typically linear path, a resulting X-ray beam sweeps through the system's beam plane across the imaged object, such that an image line may be acquired. An orthogonal image dimension is obtained either by moving the imaged object through the beam plane or by moving the beam plane across the imaged object.
The common conveyer-based inspection systems use the first approach (moving the imaged object through the beam plane). The latter (moving the beam plane across to the object) is suitable for stationary objects. Motion of the beam plane is typically achieved by one of two methods: The imaging system is moved linearly along the imaged object, or else the imaging system turns and thereby sweeps the beam plane over the imaged object in doing so.
A notable exception to the general practice of scanning within a beam plane and moving the beam plane relative to an object is described in U.S. Patent Application No. 20070172031 by Cason and Rothschild, incorporated herein by reference. The application discloses “a beam scanning device comprising: a. a first scanning element constrained to motion solely with respect to a first single axis and having at least one aperture for scanning radiation from inside the first scanning element to outside the first scanning element; and b. a second scanning element constrained to motion solely with respect to a second single axis and having at least one aperture for scanning radiation that has been transmitted through the first scanning element across a region of an inspected object”.
An imaging system for stationary objects that derives one axis of motion from rotation is conceptually simple but rotating the system, or a large part of it, is not only slow (typical image acquisition times would be many seconds) but also becomes mechanically challenging for larger, higher power systems.
Signal-to-noise and spatial resolution considerations dictate that in order to acquire two-dimensional backscatter images in a second or less, the imaging system must typically feature a high line rate and a powerful X-ray source. U.S. Pat. No. 8,576,989, assigned to Rapiscan Systems, Inc. discloses “a beam chopping apparatus, and more specifically, a helical shutter for an electron beam system that is employed in radiation-based scanning systems, and more specifically, a beam chopping apparatus that allows for variability in both velocity and beam spot size by modifying the physical characteristics or geometry of the beam chopper apparatus.”
The highest line rates are achieved by sweeping an electron beam along a linear target and collimating the emitted X-rays with a stationary aperture. Not only can the electron beam be controlled to scan the entire length of the X-ray production target in a fraction of a millisecond, moving the beam fast across the target also distributes heat generated by the impinging electron beam and thus enables focal spots of significantly higher power densities than possible in conventional X-ray tubes.
U.S. Pat. No. 6,282,260, assigned to American Science & Engineering, Inc. which is incorporated herein by reference, discloses “a hand holdable inspection device for three-dimensional inspection of a volume distal to a surface. The inspection device has a hand-holdable unit including a source of penetrating radiation for providing a beam of specified cross-section and a detector arrangement for detecting penetrating radiation from the beam scattered by the object in the direction of the detector arrangement and for generating a scattered radiation signal.”
Although conventional methods for acquiring a two-dimensional image exist, such methods do not lend themselves to fast scanning or scanning with long collimation lengths. Further, electron beam tubes with sufficiently large two-dimensional transmission targets are technically challenging and have not yet become commercially available. For high-power sources, reflection targets remain the only viable choice that can make electron beam line scanning sources practical.
Having a fast line scanner enables imaging of fast moving objects. However, for acquiring image frames of a stationary object, the beam plane must move at the desired frame rate. For sub-second image frame acquisition times, rotating the entire X-ray source and beam forming assembly is not practical or efficient.
Hence there is need for a novel method and apparatus for acquiring wide field-of-view backscatter X-ray images of stationary objects without rotating the source.